1. Field of the Invention
This invention relates to a zoom finder, and more particularly to a compact zoom finder having a zoom ratio of approximately 2. 2. Description of the Prior Art
A zoom finder whose zoom ratio is about 1.5 to 2.0 and maximum field angle is about 54.degree. has been earnestly desired for use with a compact camera, a cine-camera and a VCR camera.
In the zoom finder, a diopter must be maintained constant simultaneously with the variation of magnification, and therefore three units have to be provided at a minumum. Known zoom types includes a positive-negative-positive type (Japanese Patent Publication No. 3024/52, and Japanese Patent Application Laid-Open No. 63014/78) including a first unit, a second unit and a third unit having a positive, a negative and a positive refracting power, respectively; and a negative-positive-negative type (Japanese Patent Publication No. 9389/58) including a first unit, a second unit and a third unit having a negative, a positive and a negative refracting power, respectively.
However, in the positive-negative-positive type, the out-of-axial light is incident as convergent rays upon the second unit from the first unit, as shown in FIG. 3(a), and therefore the effective diameter of the first unit lens increases and the aperture of the finder tends to increase. On the other hand, in the negative-positive-negative type, the out-of-axial light is incident as divergent rays upon the second unit from the first unit, as shown in FIG. 3(b), and therefore the effective diameter of the first unit lens is small and the aperture of the finder can be made to be smaller.
Therefore, for use with compact cameras or the like, the zoom finder of the negative-positive-negative type is advantageous. However, in the above-described known types, the maximum magnification of the finder is above 1, and the distance between the final surface of the lens and the pupil cannot be made sufficient and the correction of aberrations is not sufficient.